This invention relates to the configuration of a data processing input/output (I/O) system, and more particularly relates to defining the accessibility of paths in the data processing I/O system.
In the prior art, a peripheral device or device path may be precluded from being accessible during system initialization for the following reasons:
Not enough resources at the control unit to accommodate the system (i.e. logical paths); PA1 link failure; PA1 internal control unit failure; PA1 dynamic switch failure; or PA1 human error. PA1 link failure; PA1 internal control unit failure; PA1 dynamic switch failure; or PA1 human error such as the disconnection of a cable resulting in a link failure.
Additionally, during the normal operation of the data processing system, I/O resources may become temporarily unaccessible due to one of the following conditions:
When the operating system tries to access a device over a path in the failure state, a not operational condition is recognized by the software and the path is marked logically offline wherein the path is no longer eligible to start I/O requests. If all paths to a device are marked offline while the system is initializing, then the device is marked offline. If all the paths to a device are marked offline while the system is running, the device is forced offline and all I/O attempts to the device are rejected with a permanent error.
If the I/O resource becomes available, no indication is given to the operating system. Thus the system operator must manually issue operator commands to put the resources back into use.
In the prior art, when a peripheral device in an I/O system became unaccessible to the I/O system it was only necessary to set a latch indicating that the device was busy. Before an I/O operation, the I/O system interrogated the latch to see if the device was accessible. As soon as the device became accessible, the latch is reset to remove the busy condition, wherein the resetting of the latch allowed the I/O operation to proceed. However, in an I/O system in which a channel is connected to a peripheral device by a serial link, especially through a switch, status of the device must be sent to the channel by means of an exchange of data frames instead of the setting of a latch. In the event that a resource such as a link between the peripheral device and the channel is broken, such as by accidentally unplugging the link, an exchange cannot take place until the link is repaired or replaced. In these situations, the peripheral device is typically isolated or "boxed" from the channel so that future I/O operations between the channel and the device are not attempted until the boxed device is manually made accessible to the I/O system by a "vary on line" process that restores the link accessibility.